1. Field of the Invention
The present invention generally relates to tank venting valves for relieving the internal pressure in tanks at a predetermined pressure. More particularly, the invention relates to pilot operated tank vent valves. Most particularly, the invention relates to a pilot operated actuator that is adaptable to fit on tank venting valves and which is designed to open the tank venting valve in event of failure.
2. Related Art
There are several types of pressure relief valves for atmospheric or low pressure storage tanks. One of the basic designs is known as a "weight loaded tank vent". The design utilizes a weighted cover over a nozzle. The cover, also known as a pallet, forms a barrier to the nozzle opening. The weight of the pallet determines at what pressure the pallet will lift open to relieve pressure in the tank. The opening pressure is known as the "set pressure". The set pressure can easily be determined by dividing the weight of the pallet assembly by the open area of the nozzle. If a higher set pressure is desired more weight is added to the pallet.
Another type of pressure relieving device is a spring loaded tank vent. The spring loaded type is used for set pressures above that which is practical for weight loaded operation due to either a high set pressure where there is not enough room for the weights on the pallet, or where the size of the vent nozzle dictates a large weight on the pallet.
A third type of tank vent is pilot operated. Pilot operated tank vents utilize the tank pressure acting on an area larger than the nozzle area to create a higher downward force to hold down a pallet or seat plate against the tank pressure. For this reason pilot operated tank vents seal tighter than weight or spring loaded vents until the set or opening pressure is reached. The set pressure is adjustable by means of a spring in the pilot.
Any of the three types may be vented to the atmosphere or piped into headers for vapor recovery or burning. It is a feature of the present pilot operated valve that it will fail in the open position in the event of component failure. The design of the actuator allows easy retrofit of most existing weight or spring loaded tank vents to be pilot operated vents in a "piggy back" fashion, which allows for easy access to the main valve, actuator or pilot for repairs or maintenance. These and other features and advantages will become apparent from the following descriptions.